When you decide to start a family, you make a/an ______ to them. You shouldn’t turn your back on the people who love you.

A．contribution　

B．commitment 　

C．devotion　

D．effort

图示结构在C点作用一水平力P，其大小等于300kN。设AC杆与铅直线的夹角为a，该杆最多只能承受150kN。若要结构不致破坏，角度a的大小应为：（）

A.a=0°

B.a=30°

C.a=45°

D.a=60°